Among EK, ENa, and ECa, which is the most negative value?

• A. EK
• B. ENa
• C. ECa
• D. All are positive

Answer: (A)

This tests the idea of ion equilibrium potentials—the voltage at which there is no net flow of a specific ion across the membrane, set by its concentration gradient and charge. Potassium has a high inside concentration and low outside, and it’s a monovalent cation, so the potassium equilibrium potential (EK) is negative, typically around -90 mV. Sodium has a large outside-to-inside gradient and is also monovalent, yielding a positive equilibrium potential around +60 to +70 mV. Calcium is divalent and experiences a very large gradient, giving an even more positive equilibrium potential, about +120 mV. Since -90 mV is more negative than +60 to +70 mV and +120 mV, EK is the most negative value among them. The resting membrane potential in many cells tends toward EK because the membrane is more permeable to K+ at rest, but the key point is the relative signs and magnitudes from these equilibrium potentials.